The telescope of optical geodetic instruments, such as theodolites, transits, levels, and the like, comprises a reticle located on the optical axis by means of which an operator establishes a datum for subjects under observation. Although such reticle elements may be located substantially on the optical axis during manufacture of the telescope, it is essential that means be provided for adjustment of the position of the reticle more precisely subsequent to assembly of the instrument. It is not uncommon for effective operation of geodetic telescopes to depend upon the maintenance of reticle adjustment within a range about 1-2 micrometers.
Such reticle adjusting means have remained virtually unchanged for many years and are typically as represented in U.S. Pat. No. 2,937,570, or more specifically shown in FIG. 1 of the instant drawings. As there depicted, a prior art reticle adjustment comprises a reticle mount 13 in which reticle glass 12 is permanently affixed by means of adhesive or the like. The mount is situated within the telescope body 11 where it is adjustably held by four diametrically opposed screw members, such as capstan screws 15, received in tapped bores 14 in the body of mount 13. Screws 15 communicate with mount 13 through access ports 17 in the wall of telescope body 11. Shutters 16 provide flat seats for the shoulders of screws 15 and further provide means for sealing ports 17, thus protecting the interior of the telescope from environmental hazards such as dust, moisture, and the like.
Although this previous form of reticle adjustment has served quite well in effecting rotational adjustment of the reticle, it has suffered from an inherent instability with respect to lateral adjustment. The cause of this instability would appear to be such as is schematically represented in FIGS. 1a and 1b. As is seen in FIG. 1a, rotational adjustment of the reticle, which is accomplished by loosening at least two adjacent ones of screws 15 and rotating the mount/screw assembly as a whole, results in minute displacement of the axes of screws 15 from an original position represented by the solid lines in FIG. 1a to a new position represented by the broken lines. During such a rotational adjustment, however, the point of intersection of the axes of the screws remains fixed at the mechanical center 18 of the telescope body and retightening of the screws to again lock the mount and reticle in a fixed position has no effect on the stability of the system.
On the other hand, a lateral adjustment of the reticle, accomplished by loosening the adjustment screws and rotating opposing pairs of such screws to shift the position of mount 13, results in displacement of the screw axes, as shown by the dotted lines in FIG. 1b, with simultaneous displacement of the point of intersection 19 of these axes from the center 18 of the telescope body. Although it would appear that the final tightening of the adjustment screws would effect an immobility in this position of the reticle mount, there in fact exists, probably due in part to the curvature of the shutter elements 16, a stress in the assembly tending to urge the screw axes back toward the center 18 of the telescope body. This instability, combined with environmental variations and the mechanical shock to which geodetic instruments are often exposed, results in the need for repeated readjustment of the reticle of instruments comprising these prior art types of adjusting means.
As will be seen in greater detail in the following description, the reticle adjusting means of the present invention eliminates the instability which has heretofore made optical geodetic instruments so susceptible to misalignment during the course of ordinary use and handling.